This invention relates to an ultrasonic diagnostic apparatus for extracting and displaying the motion component of the examined region of an examined body (patient), for example, a locomotive region such as a blood vessel or the heart by utilizing an ultrasonic beam, and more particularly to an ultrasonic diagnostic apparatus for displaying both said locomotive regions which move quickly and which move slowly in an easily observable state.
As described in JP-A-62-189054, a conventional ultrasonic diagnostic apparatus of this kind comprises a beam transmission/reception unit (a probe and an ultrasonic beam transmission/reception unit) for transmitting and receiving an ultrasonic beam to and from the examined body, a tomographic scanning unit for obtaining tomographic image (B-mode image) data about the examined body including the locomotive regions repeatedly at specified intervals by using reflected echo signals from the ultrasonic beam transmission/reception unit, a unit (subtraction processor) for calculation between time-series images obtained by the tomographic scanning unit, and an image display device (TV monitor) for displaying subtraction image data from the subtraction image data generating unit.
Therefore, from the reflected echo signals from the ultrasonic beam transmission/reception unit, tomographic image data of the examined body, including the locomotive regions is obtained by the tomographic scanning unit repeatedly at specified intervals, calculation is performed between time-series images obtained by the tomographic scanning unit to generate subtraction image data, and the subtraction image data is displayed on the image display device.
In this case, in said time-series images, if the locomotive region in the examined body moves in a time space from when an image is obtained in the previous scanning until an image is obtained in the current scanning, a difference occurs in the image data between the previous and current images of the region that moved. The image data on those two images is the same for the regions which have been stationary and subtraction data is zero, so that only that locomotive region is displayed.
In such a conventional ultrasonic diagnostic apparatus, however, two completed tomographic images of different time phases are taken out, a subtraction operation is performed between images generate subtraction image data, and a subtraction image is displayed. Therefore, the time required for subtraction depends on the frame rate of image display. For example, in the case of 30 frames/sec, if a subtraction operation is performed between two successive images, the time for subtraction is about 33 ms, and if subtraction is performed between images separated by one image, the subtraction time is about 66 ms, and if subtraction is performed between images separated by two images, the subtraction time is about 99 ms.
In this case, as the time interval between two tomographic images subjected to a subtraction operation increases, the amount of displacement of the movement of the fast-moving locomotive region increases, so that the detailed movement of that locomotive region cannot be observed in detail. Conventionally, when the shortest time interval is selected, the subtraction time is about 33 ms. But with the fast-moving regions, such as the valves or walls of the heart, this time interval is still so long that the movement of the region at issue cannot sometimes be observed in detail. In order to observe the high-speed locomotive region such as the valves and walls of the heart, the subtraction time is preferably about 10 ms at the frame rate of about 100 frames/sec. However, such a fast imaging is impossible with ordinary ultrasonic diagnostic apparatuses.
On the other hand, with the slow-moving locotomive regions in which their amounts of displacement is small, when the subtraction time is very short, the moving condition can hardly be observed. In this case, it is necessary to elongate the subtraction time adequately to make their moving condition observable.